This invention relates to rolling element bearings and, more particularly, to high speed bearings of the type used in turbomachinery.
In general, bearings used in turbomachinery such as gas turbine engines must withstand environmental and operating conditions much more strenuous than do bearings found in many ordinary applications. As a result, special bearing steels were developed for turbomachinery and have performed well for many years. One of the most commonly used materials for bearings in gas turbine engines manufactured in the United States is a commercially available iron-base alloy identified as AISI M-50 for which there are published specifications AMS6490 and AMS 6491. Similar steels, though more highly alloyed, are used in European gas turbine engine manufacture, for example commercially available alloy T1 (18-4-1). Published data indicates little difference between these two materials in regard to rolling contact fatigue or fracture toughness.
Alloy M-50, which is a Cr-Mo-V high speed tool steel, has been used for about 25 years in bearings for both commercial and military aircraft gas turbine engines. However, with the development of higher speed, higher temperature advanced gas turbine engines, it has been recognized that a bearing member of improved life is required. In such advanced designs, operational conditions have placed new demands on bearings as a result of increase in engine speeds to achieve improved operating efficiency and lower fuel consumption. Such increases in engine speed results in an increase in the bearing DN (the bearing bore in millimeters times the shaft speed in revolutions per minute), and an increase in operating hoop tensile stress. As the tensile stress increases, the low fracture toughness, and more specifically the low stress intensity factor, of current through-hardened rolling element bearing materials becomes a technical barrier which can be critical to the operation of such advanced engines. The potential significant decrease in bearing reliability using current bearing materials has resulted in design limitations for such advanced engines. In addition, the higher stress levels in such engines are sufficient to cause race fractures following rolling contact fatigue initiated spalling failure, which may compromise engine integrity.